Depositional processes in latest Pleistocene and Holocene ephemeral streams of the Main Ethiopian Rift (Ethiopia)

摘要

Discontinuous ephemeral streams were described originally from the southwest USA as consisting of alternating reaches of incised rectangular channels and aggradational channel-fan systems. Modern ephemeral streams on the flanks of the Gademota Ridge, in the Lake Region of the Main Ethiopian Rift, show a similar pattern, and deposits exposed in the walls of modern stream valleys record the development of these systems since the latest Pleistocene. Streamflow, sheetflow and massflow deposits are recognized to be the basic sedimentary facies in palaeovalley fills, and within palaeochannel–fan systems. These deposits are stacked in a cyclic pattern within multiple and single valley fills, with basal streamflow deposits overlain by hyperconcentrated sheetflow and eventually by debris-flow deposits. Debris-fall and debris-flow deposits related to bank failure occur interbedded with streamflow and sheetflow facies. A model for the cyclic incision and aggradation of the latest Pleistocene-Holocene ephemeral streams in this area is proposed based on incision during transition to a more moist climate, followed by aggradation during two main genetic stages that contrast in discharge volumes. During the early stage, relatively high and continuous seasonal discharge transports coarse-grained material, and covers the incised channel base with gravelly and sandy dune-scale bedforms. Bank undercutting and piping produce instability of the channel margins, with accumulation of relatively thick aprons of fine-grained material. The channel fan system is supplied with pebbly–sandy material flushed from the valleys. During the late stage, a decreased volume of flashy discharge redistributes sediment stored in the bank aprons within the valleys and to the next downstream channel-fan system through hyperconcentrated sheetflows and debris flows. Spreading of debris flows and sheetflows on interfluves marks the overfilling of the valleys.